This is going to sound terrible, but there is no offence unless there is an offence. For instance, if he forges documents in support of his identity, then he may be committing forgery. Merely walking into a room and claiming to be a person you're not, if it's a fictitious person, and then proceeding to have a conversation based on a fictitious identity, is not a crime.
On September 17th, 2009. See this statement in context.